RESUMEN
BACKGROUND: Fractional excretion of magnesium (FEMg) is commonly used to diagnose of renal magnesium (Mg) wasting, but it can be affected by serum Mg (SMg) and serum creatinine concentration (SCr). We investigated the sensitivity and specificity of FEMg to diagnose Mg wasting in subgroups with different SMg and eGFR (estimated glomerular filtration rate) in pediatric nephrology practice. METHODS: One hundred and nineteen patients (59 males and 60 females, median 15â¯years) seen in our pediatric clinic were investigated for FEMg, SMg, eGFR, and urine Mg-to-creatinine ratio (Mg/Cr). Normal eGFR was defined asâ¯≥â¯90â¯ml/min/1.73â¯m2 or for infants SCrâ¯<â¯chronic kidney disease stage 2. Urine Mg/Cr was compared with age-specific reference values. RESULTS: Sixteen of all patients (13â¯%) had hypomagnesemia. All had FEMg greater than the cut-off value of 2â¯%. Only 4 patients had elevated urine Mg/Cr. Of 65 patients with normal SMg and eGFR, 19 had FEMg above the cut-off value of 4â¯%. Of these, 13 patients had elevated urine Mg/Cr i.e. Mg wasting (sensitivity and specificity of FEMg, 93â¯% and 88â¯%, respectively). Among 38 patients with normal SMg and low eGFR, 30 had FEMgâ¯>â¯4â¯%, but only 6 had elevated urine Mg/Cr (sensitivity 100â¯% and specificity 25â¯%). Overall, hypomagnesemic patients and normomagnesemic patients with elevated urine Mg/Cr were diagnosed with Mg wasting (36/119, 30â¯%). CONCLUSIONS: FEMg has variable sensitivity and specificity depending on SMg and eGFR in the diagnosis of Mg wasting. Mg wasting is not uncommon in pediatric nephrology practice.
